I'm playing a monk for the first time and I'm curious as to why anyone would even use monk weapons when unarmed damage is so much better? I'd like to use sai's for flavour but I only ever get 1d4 with them, even though you use them very similarly to fists (blunt). Is there a way to make monk weapons matter or is it best to just stick with fists?

3 Answers
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There are multiple reasons a monk might use a monk weapon instead of their unarmed strikes.

You've taken an archetype that replaces your unarmed strike damage or allows you to deal damage with another weapon as if you were unarmed,

You are more focused on battlefield control than damage and prefer a weapon with the disarm, trip, reach, grapple, or brace quality.

You have another, more unusual focus and prefer a weapon with the performance, distracting, or blocking quality,

You have a specific build that benefits from other weapon quality or statistic (higher critical range or damage, special material you can't replicate with your fists),

You are fighting a creature with a DR that you can overcome with a monk weapon but not your unarmed strikes (most likely if you've looted an enchanted monk weapon, but carrying a S/P weapon made of a special material is a good idea anyway)

You are fighting a creature that damages you if struck with an unarmed strike,

You are fighting an evasive creature and need the range of a thrown weapon,

You have been swallowed whole and need a cutting weapon.

I nearly forgot this one: at low levels, some monk weapons do more damage than your unarmed strikes anyway. I think one of them is a d10, which you don't tie until 8th or beat until 12th.

At low levels, monk weapons are cheaper to enhance. For one example, you can buy silvered or cold iron monk weapons for a cheaper cost than Greater Magic Fang. This increases your damage against creatures that are affected by silvered or cold iron weapons. So, if you are fighting a creature with DR 5/silver (for example) the silvered weapon provides an improvement.

You can also buy Masterwork weapons. This gives a bonus which can help at lower levels. You can't buy masterwork fists. :-)

Unarmed Strike is a Bludgeoning type attack. If you want to bypass DR with Piercing or Slashing attacks, a weapon gives you more options.

Shuriken provide a way to make a ranged attack using flurry of blows

Weapons in two hands provide 1.5 your STR bonus, which helps damage. While the Power Attack feat also helps increase your damage, it needs a +1 BAB so it is not available at first level.

Very early on in the game you can get a Masterwork version of your sword (either buy a MwK version of it, Masterwork Transformation spell or Exemplar Weapon Salve) this gives you a +1 to hit early, much cheaper than an amulet of mighty fists and frees up the necks slot for more important items. Even at early levels neck slot items like Talisman of Beneficial Winds for 50gp can save you from a fatal fall

Temple Sword can be made of special materials that your unarmed-strike cannot, things such as lead-lined

Temple Sword has a better crit range and can better facilitate a crit-focused build as a weapon can get the Keen quality and Chthonic Steel

If you're a medium character you can then get a Large sized Temple Sword (By purchase or Resize Item) which does 2d6 damage and can be wielded always as a 2 handed weapon without penalty with Effortless Lace. Not till Monk lv 16 does unarmed strike do more damage and even then it's without 1.5x strength bonus

If you can get the spell Lead Blades, this further stacks to 3d8 damage

Weapons can have very valuable special abilities like Spell Storing or Bewildering into a physical weapon that unarmed strikes cannot

You aren't directly touching your target on a successful melee attack

You can still use stunning fist with Monk Melee Weapons with the feat Cornugon Smash

you now have slashing damage and bludgeoning damage, able to switch between then with either slicing with sword of kicking

Amazingly good spells like Dazzling Blade (essentially get Quickened Spell to blind as a stock level 1 spells) can only apply to metal weapons.

Your damage output can be truly massive, potential of wiping out a Greater Barghest in a single full-attack.

Also consider Shurikens:

Most classes lack proficiency in this weapon so most overlook its unique combination of being thrown ammunition so you can draw as a free action and throw. This means you can easily extend your full round attack by substituting in a shuriken throw any time instead of a melee attack.

As Shurikens are ammunition they are more useful for their enhancement qualities. As ammunition is enhanced in a group of 50 therefore a single unit of ammunition would only cost one 50th of a whole enhancement. So if you only occasionally want a magic weapon ability like Bewildering you can get it in a single Shuriken form.

Also of note, if facing incorporeal foes they are best kept a distance from and dealt with using Ghost Salt. Ghost Salt goes much further over ammunition for 10 units, 10 attacks.